Menopausal hot flashes are thought to be a disorder of thermoregulation initiated centrally within the medial preoptic area of the hypothalamus. These heat-loss mechanisms appear to be activated in the presence of normal core body temperature. Previous studies have demonstrated that thermal stimuli have the potential to alter sleep stages. We performed 24-hour ambulatory recordings of hot flashes and all-night sleep parameters on 12 postmenopausal women with hot flashes and seven postmenopausal women without flashes to determine whether the presence of hot flashes prior to sleep or during sleep itself would result in alterations in sleep pattern. The results show that hot flashes are associated with increased Stage 4 sleep and a shortened first rapid eye movement period. Hot flashes occurring in the 2 hours prior to sleep onset were positively correlated with the amount of slow-wave sleep. The central thermoregulatory mechanism underlying hot flashes may affect hypnogenic pathways inducing sleep and heat loss in the absence of a thermal load.